Annex
Measurement methods
In order to be able to quantify whether or not there is an interference problem it is necessary to be able to make repeatable and reliable measurements. The quantities to be measured are:
- The level of interference signals that are likely to cause the user varying degrees of difficulty when using the hearing aid in the presence of a mobile phone.
- The level of immunity of the hearing aid to specified types of signal over a defined frequency range
- The level of radiation from the mobile phone that is present at the hearing aid when it is being used in a normal manner.
In order to establish the levels of interference signals that are likely to cause a problem and to establish a measurement method subjective evaluations have been undertaken by the European Hearing Instrument Manufacturers Association (EHIMA) in 1995 [12], and by the National Acoustics Laboratory, Sydney, Australia also in 1995) [13]. These evaluations related the level of the interference signal to an equivalent acoustic input to the microphone of the hearing aid, now known as the Input Related Interference Level (IRIL). The level that was determined from the EHIMA study was 55 dB SPL, which represented a level of interference that was such that it did not significantly effect the perception of speech. It does not represent a zero level of interference. This level is then taken as the on that should not be exceeded if the user is to experience a satisfactory listening situation.
Given that there is a target value for IRIL there has to be an objective method of measuring the immunity of the hearing aid. This is achieved by placing the hearing aid in an appropriate RF field and measuring the acoustic output of the hearing aid. The gain of the hearing aid is also measured at the same time, with the same settings of the aid when in the RF field. The level of interference present at the output of the aid can then be equated to an equivalent acoustic input to the microphone by subtracting the value of the gain of the aid from the output signal and quoting it as the IRIL. The method of measurement was proposed in the EHIMA (1995) [12] study and has since been incorporated in the IEC 60118-13 standard, IEC (1997) [6].
The level of radiation from the mobile phone will obviously directly influence the level of interference in the hearing aid. A method of measurement of the level of radiation from a mobile phone relevant to hearing aid interference has been proposed by the American National Standards Institute (ANSI) and recently accepted as an American national standard. The standard, ANSI P63.19 (2001) [7], requires a matrix of measurements around the ear cap of the mobile phone, which effectively plots the level of field strength that may affect a hearing aid worn behind the ear when the mobile phone is used in the normal manner.
Consequently there has been available for some while agreed methods of measurement both of the immunity of hearing aids and the level of radiation from mobile phones. All major European manufacturers of hearing aids have been using the above methods of measuring immunity to improve the level of immunity of their products. Mobile phone manufacturers have not adopted the use of the ANSI method of specifying levels of radiation and consequently hearing aid users have no means of knowing which mobile phones have high levels of radiation relevant to interference problems and those that have low values.
The IEC 60118-13 (1997) [6] standard provided a means of measuring the immunity of hearing aids and also specified two scenarios in which levels of immunity were to be specified. These were the so-called “Bystander” and the “User” situation. The “Bystander” situation was where a hearing aid user might be close to a person using a mobile phone and receive an interfering signal. A signal strength of 3.5V/m was assigned to this situation for the agreed IRIL of 55 dB SPL. Consequently all new production hearing aids meet this requirement.
However the IEC 60118-13 [6] standard did not specify a value of field strength for the “User” situation but a proposed revision of this standard now gives a value of 75V/m.
The ANSI C63.19 [7] standard uses the same methods for measuring the acoustic output of the hearing aid under test but gives a set of rating values for both the immunity of the hearing aid and the radiation from the hearing aid. This in theory then allows hearing aids and mobile phones to be matched for immunity and radiation levels to give a user situation that is acceptable.
Test results
The methods of measuring immunity described by IEC and ANSI are very similar except for the method of generating the RF field. The IEC standard recommends the use of a GTEM cell whereas the ANSI standard proposes the use of a dipole. The problem that currently exists is that the correlation between these two types of measurement is not good and furthermore the correlation with user experience is not well documented. Currently work is being undertaken by EHIMA in conjunction with members of the ANSI committee to try and resolve these problems.
Both objective methods of measurement do not take into account the effect of the hearing aid being worn on a real head. It is understood that the further the aid is from the mobile phone antenna the lower the level of interfering signal. Consequently it is expected that hearing aids worn in the ear, in the ear canal and deep in the canal will be less affected by interference from the mobile phone.
Consequently there is a need to have better knowledge of the relationship between user experience and the objective measurements of immunity. Currently the best advice that can be given to a hearing aid user, in the absence of any data readily available to them, who wants to use a mobile phone in the normal manner, is to try various phones and see which one gives the least interference. Taking care, of course, that the phone is radiating at a level that is typical of its normal use in the user environment.
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